The autoregulation of glomerular filtration is, by necessity, intimately related to the regulation of tubular reabsorption of NaCl and water. We plan to extend our previous studies by examining and defining the specific mechanisms which interrelate the processes of glomerular filtration and tubular reabsorption. Utilizing micropuncture techniques as previously described, we plan to delineate the mechanisms whereby the rate of tubular reabsorption contributes to the regulation of nephron filtration (tubulo-glomerular feedback). A variety of studies will delineate the specific determinants of glomerular ultrafiltration which constitute the efferent limb of the feedback system. Other studies will further examine potential humoral mediators of tubulo-glomerular feedback, such as the renin-angiotensin system, and define the influence of these substances on both filtration and reabsorption utilizing antagonists of their activity. The second general type of study will examine the juxtaposed control system; the mechanisms whereby absolute reabsorption in the proximal tubule changes in proportion to variations in nephron filtration rate (glomerular-tubular balance). Utilizing tubulo-glomerular feedback mechanisms to change nephron filtration rate and peritubular "physical factors" and in vivo microperfusion of the proximal tubule and distal nephron, we will define the respective roles of intraluminal influences and extraluminal "physical factors" in mediating glomerular tubular balance. Also in other studies, we will analyze the respective influences of load (sngfr) and changes in renal interstitial pressure upon the changes in proximal tubular absolute reabsorption. From the results of these examinations we hope to further define the mechanisms whereby glomerular filtration rate and tubular reabsorption interrelate to maintain volume homeostasis.